The following relates to the nuclear power reactor arts and related arts.
With reference to FIGS. 1 and 2, the lower portion of a nuclear power plant of the pressurized water configuration, commonly called a pressurized water reactor (PWR) design, is shown. A nuclear reactor core 10 comprises an assembly of vertically oriented fuel rods containing fissile material, typically 235U. The reactor core 10 is disposed at or near the bottom of a pressure vessel 12 that contains primary coolant water serving as a moderator to moderate the chain reaction and as coolant to cool the reactor core 10. The primary coolant further acts as a heat transfer medium conveying heat generated in the reactor core 10 to a steam generator. At the steam generator, heat from the primary coolant transfers to a secondary coolant loop to convert the secondary coolant into steam that is used for a useful purpose, such as driving a turbine of an electrical power generation facility. A conventional PWR design includes one or (typically) more steam generators that are external to the pressure vessel containing the nuclear reactor core. Large-diameter piping carries primary coolant from the pressure vessel to the external steam generator and back from the steam generator to the pressure vessel to complete a primary coolant flow loop. In some designs the external steam generator is replaced by an internal steam generator located inside the pressure vessel, which has the advantage of eliminating the large diameter piping (replaced by secondary coolant feedwater and steam outlet lines that are typically of lower diameter and that do not carry the primary coolant that flows through the reactor core). Note that FIG. 1 is a diagrammatic view of the lower reactor core region and does not include features relating to the steam generator or ancillary components.
The vertical fuel rods of the reactor core 10 are organized into fuel assemblies 14. Illustrative FIG. 1 shows a side view of a 9×9 array of fuel assemblies 14, although arrays of other sizes and/or dimensions can be employed. In turn, each fuel assembly 14 comprises an array of vertically oriented fuel rods, such as a 18×18 array of fuel rods, or a 14×14 array, or so forth. The fuel assemblies further include a lower end fitting, upper end fitting, vertical guide tubes connecting the end fittings, and a number of spacer grids connected to the guide tubes, instrument tubes and fuel rods. The spacer grids fit around the guide tubes to precisely define the spacing between fuel rods and to add stiffness to the fuel assembly 14. The spacer grids may or may not be welded to the guide tubes. (Note, FIGS. 1 and 2 represent the fuel rods of each fuel assembly 14 are shown diagrammatically with vertical lines which are not to scale respective to size or quantity, and the spacer grids, guide tubes, and other features are not shown). It is noted that the dimensions of the array of fuel assemblies 14 may in general be different from the dimensions of the array of fuel rods within the fuel assembly 14. The fuel assemblies may employ rectangular fuel rod packing and have a square cross section, or may employ hexagonal fuel rod packing and have a hexagonal cross section, or so forth). The reactor core 10 comprising fuel assemblies 14 is disposed in a core basket 16 that is mounted inside the pressure vessel 12. The lower end fitting of each fuel assembly 14 includes features 18 that engage with a core plate. (The core plate, basket mounting, and other details are not shown in diagrammatic FIG. 1).
The reactor control system typically includes a control rod assembly (CRA) operated by a control rod drive mechanism (CRDM) (not shown in FIGS. 1 and 2). The CRA includes vertically oriented control rods 20 containing neutron poison. A given control rod is controllably inserted into one fuel assembly 14 through a designated vertical guide tube of the fuel assembly 14. Typically, all the control rods for a given fuel assembly 14 are connected at their top ends to a common termination structure 22, sometimes called a spider, and a connecting rod 24 connects at its lower end with the spider 22 and at its upper portion with the CRDM (upper end not shown). The CRA for a single fuel assembly 14 thus comprises the control rods 20, the spider 22, and the connecting rod 24, and this CRA moves as a single translating unit. In the PWR design, the CRA is located above the reactor core 10 and moves upward in order to withdraw the control rods 20 from the fuel assembly 14 (and thereby increase reactivity) or downward in order to insert the control rods 20 into the fuel assembly 14 (and thereby decrease reactivity). The CRDM is typically designed to release the control rods so as to fall into the reactor core 10 and quickly quench the chain reaction in the event of a power failure or other abnormal event.
Because the reactor control system is a safety-related feature, applicable nuclear safety regulations (for example, promulgated by the Nuclear Regulatory Commission, NRC, in the United States) pertain to its reliability, and typically dictate that the translation of the CRA be reliable and not prone to jamming. The translation of the CRA should be guided to ensure the control rods move vertically without undue bowing or lateral motion. Toward this end, each CRA is supported by a control rod guide structure 30 which comprises horizontal guide plates 32 mounted in a spaced-apart fashion on vertical frame elements 34. Each guide plate 32 includes openings or passages or other camming surfaces (not visible in the side view of diagrammatic FIGS. 1 and 2) that constrain the CRA so that the rods 20, 24 are limited to vertical movement without bowing or lateral movement.
With continuing reference to FIGS. 1 and 2, the CRA guide assemblies 30 have substantial weight indicated by downward arrow FG,weight in FIG. 2, and are supported by a weight-bearing upper core plate 40. The fuel assemblies 14 are also relatively heavy. However, in a conventional PWR the primary coolant circulation rises through the fuel assemblies 14, producing a net lifting force on the fuel assemblies 14 indicated by upward arrow FFA,lift. Accordingly, the fuel assemblies 14 while typically resting on the bottom of the core basket 16, are susceptible to being lifted upward by the lift force FFA,lift and press against the upper core plate 40. The lift force FFA,lift is thus also borne by the upper core plate 40. The upper core plate 40 thus is a spacer element disposed between and spacing apart the lower end of the CRA guide assembly 30 and the upper end of the corresponding fuel assembly 14. To avoid damaging the fuel rods, each fuel assembly 14 typically includes a hold-down spring sub-assembly 42 that preloads the fuel assembly 14 against the upper core plate 40 and prevents lift-off of the fuel assembly 14 during normal operation. The hold-down spring 42 is thus also disposed between the lower end of the CRA guide assembly 30 and the upper end of the corresponding fuel assembly 14. Additionally, alignment features 44, 46 are provided on the upper end of the fuel assembly 14 and the lower end of the CRA guide structure 30, respectively, to assist alignment.
A PWR such as that of FIGS. 1 and 2 is typically designed to provide electrical power of around 500-1600 megawatts. The fuel assemblies 14 for these reactors are typically between 12 and 14 feet long (i.e., vertical height) and vary in array size from 14×14 fuel rods per fuel assembly to 18×18 fuel rods per fuel assembly. The fuel assemblies for such PWR systems are typically designed to operate between 12- and 24-month cycles before being shuffled in the reactor core. The fuel assemblies are typically operated for three cycles before being moved to a spent fuel pool. The fuel rods typically comprise uranium dioxide (UO2) pellets or mixed UO2/gadolinium oxide (UO2—Gd2O3) pellets, of enrichment chosen based on the desired core power.